1. Field of the Invention
This invention relates to ultrasonic imaging systems. In a primary application the invention relates to dynamically-focused receiver systems using arrays of transducers and time-varying delay lines.
2. Description of Prior Art
Ultrasonic imaging techniques are widely used for non-destructive testing in industry and for visualizing soft tissue structures in medical diagnosis. One of the principal difficulties with the existing instruments stems from the use of non-focused patterns in the transmit and receive modes. Thus the lateral resolution is determined primarily by the transducer size and by diffraction spreading. This results in a fundamental compromise in the size of the transducer aperture. A large transducer results in a uniform collimated beam pattern with relatively poor lateral resolution throughout the depth of the object. A small transducer results in good lateral resolution at small depths but, due to significant diffraction spreading, results in significant deterioration at the longer depths. Most transducer apertures are about one cm. in extent as a resolution compromise.
In an effort to provide improved lateral resolution in B scan imaging, lens systems are often used which improve the lateral resolution at the region of focus but not elsewhere. When using transducer arrays, electronic focusing can be accomplished by dynamically varying the delay applied to each transducer signal as the ultrasonic wave propagates. One such system has been described by G. Kossoff in "An Historical Review of Ultrasonic Investigations at the National Acoustic Laboratories," J. Clinical Ultrasound, Vol. 3, p. 39, March 1975. In one of his systems Dr. Kossoff uses a concentric ring array with time-varying delay lines connected to each ring to provide dynamic focus. This was also described at the meeting of the American Association for Ultrasound in Medicine in Seattle, Washington in October 1974. This system is not capable of electronic deflection and is therefore mechanically scanned to produce a two-dimensional image. A system using a linear transducer array with electronic deflection and focusing was described by F. L. Thurstone in "A New Ultrasound Imaging Technique Employing Two-Dimensional Electronic Beam Steering," Acoustical Holography, Vol. 5, Plenum Press. Here switched delay lines are connected to each transducer output and used to provide beam deflection and dynamic focus.
In both of these systems the delay elements represent a significant problem. They are noisy, difficult to rapidly switch, bulky, and require a relatively large delay range. In order to solve many of these problems the CCD (charge coupled device) is used as the analog variable delay element because of its small size and simplicity. It makes possible a hand-held ultrasonic probe, which is important to physicians. The use of CCD's in this dynamic focusing application is described by J. Shott and R. Melen in "The Rozorback CCD: A High-Performance Parallel Input Delay Line Architecture," Proc. of 1975 Int. Solid State Circuits Conf., pp. 150-151, February 1975. Although the CCD delay elements solve many of the problems, the large depth range required in ultrasonic imaging systems requires a correspondingly large range in clock frequencies. Since the lowest clock frequency is set by the signal sampling rate, the highest frequency can become greater than the capability of CCD technology.
Another system using variable delay elements for dynamic focusing is described in U.S. Pat. No. 3,918,024 issued to A. Macovski, entitled, "Ultrasonic Array for Reflection Imaging." In this system a weighted ring array is used as the transmitter to provide good resolution in one dimension at all depths. The electronic focus is accomplished by time-varying delay lines such as CCD's with controlled clock frequencies. These delay lines, however, require a large range in order to focus over the desired depth range. This range, as previously mentioned, may well be beyond that presently available with CCD's. In addition to electronic focusing, this patent described methods to provide electronic deflection by using an array of controlled delay elements which are changed following each scan line so as to provide a sector scan.
The present invention is directed toward reducing the delay range for any variable delay system, including the CCD's. A paper based on the application of this invention to reducing the delay range using CCD's is, "CCD Dynamically Focused Lenses for Ultrasonic Imaging Systems," by R. D. Melen, J. D. Schott, J. T. Walker and J. D. Meindl, Proc. of the International Conference on Applications of CCD, San Diego, Oct. 29-31, 1975.